Transition State Theory

11 min read Updated Mar 26, 2026

If collision theory is the “bird’s-eye view” of reactions (molecules smashing into each other), transition state theory zooms in to show exactly what happens during the collision itself. It tracks the energy of the system as reactants transform into products, step by step.

The Transition State (Activated Complex)

The transition state is the highest-energy point along the reaction pathway. At this point, old bonds are partially broken and new bonds are partially formed. The transition state is denoted by the symbol ‡ (double dagger).

Key properties of transition states:

They exist at the energy maximum on the reaction coordinate diagram
They are theoretical structures that cannot be isolated or observed directly
They have an extremely brief lifetime (on the order of femtoseconds)
Once formed, they can proceed forward to products OR fall back to reactants

Reading Reaction Coordinate Diagrams

Reaction coordinate diagram showing reactants A+B at an initial energy level, rising to a transition state peak with activation energy Ea labeled, then descending to products C+D at a lower energy level, with delta H shown as the energy difference between reactants and products — A reaction coordinate diagram for an exothermic reaction. The peak represents the transition state, $E_{a}$ is the activation energy, and ΔH is the enthalpy change. *Credit: OpenStax Chemistry 2e, CC BY 4.0*

A reaction coordinate diagram contains several important pieces of information:

Activation energy ( $E_{a}$ forward): The energy difference between the reactants and the transition state peak. This is the barrier the forward reaction must overcome.

Activation energy ( $E_{a}$ reverse): The energy difference between the products and the transition state peak. This is the barrier the reverse reaction must overcome.

Free energy of reaction (ΔG): The energy difference between products and reactants. This determines whether the reaction is exergonic or endergonic.

Exergonic vs. Endergonic Reactions

Comparison of exothermic (solid black line, products lower than reactants) and endothermic (dashed red line, products higher than reactants) reaction coordinate diagrams, with delta H labeled for each, showing that both reactions pass through the same high-energy transition state — Exothermic reactions (black, solid) release energy: products are lower than reactants, ΔH is negative. Endothermic reactions (red, dashed) absorb energy: products are higher, ΔH is positive. Both must overcome the activation energy barrier at the transition state. *Credit: Wikimedia Commons, CC BY-SA 3.0*

Feature	Exergonic	Endergonic
ΔG	Negative	Positive
Products vs. Reactants	Products lower energy	Products higher energy
Spontaneous?	Yes (thermodynamically favored)	No (requires energy input)
$E_{a}$ forward vs. $E_{a}$ reverse	$E_{a}$ forward < $E_{a}$ reverse	$E_{a}$ forward > $E_{a}$ reverse

The Relationship Between $E_{a}$ and ΔG

A critical concept: activation energy and ΔG are independent. A reaction can be exergonic (thermodynamically favorable) but still have a high activation energy (kinetically slow). Diamond converting to graphite is thermodynamically favorable but kinetically so slow that your engagement ring is safe.

On the diagram:

ΔG = $E_{\text{products}}$ - $E_{\text{reactants}}$ (where on the y-axis the reaction ends vs. starts)
$E_{a}$ = $E_{\text{transition}}$ state - $E_{\text{reactants}}$ (how high you must climb)

Changing one does not automatically change the other. You can lower $E_{a}$ (with a catalyst) without changing ΔG.

Intermediates vs. Transition States

For multi-step reactions, the reaction coordinate diagram has multiple peaks and valleys:

Feature	Transition State	Intermediate
Energy position	Local maximum (peak)	Local minimum (valley)
Can be isolated?	No	Sometimes (briefly)
Stability	Maximally unstable	Relatively more stable
Symbol	‡ (double dagger)	Usually written as a chemical formula
Appears in overall equation?	No	No
Lifetime	Femtoseconds	Longer (but still short)

An intermediate sits in a valley between two peaks. It is a real molecule with a finite lifetime, formed after one elementary step and consumed in the next.

Six reaction coordinate diagrams showing progressively more complex mechanisms: single step exothermic, two-step with one intermediate, two-step with different rate-determining steps, and three-step mechanisms with two intermediates labeled I1 and I2 — Reaction coordinate diagrams for multi-step mechanisms. Top row: one-step, two-step (with intermediate I), and two-step with a different rate-determining step. Bottom row: three-step mechanisms with two intermediates (I₁ and I₂). Peaks are transition states; valleys are intermediates. *Credit: Wikimedia Commons, CC BY-SA 4.0*

On a reaction coordinate diagram, what does the highest peak represent?

Click to reveal answer

The transition state of the rate-determining step. The highest peak has the largest activation energy barrier and corresponds to the slowest step in the mechanism. The rate of the overall reaction is determined by this step.

How do you distinguish an intermediate from a transition state on a reaction coordinate diagram?